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Loki is, of course, from the comics. He plays the same role as he does in the films, he's Thor's trouble-making brother.

In a recent interview, the film's director, Joss Whedon, confirmed that the villains working with Loki in the film are from the comics, specifically the Ultimate imprint:

The alien race are the Chitauri -- or a version of them -- because they are not one of the key races and they don’t have a storied history

The Chitauri are the Ultimate universe's version of the Skrulls, sort of. As the Wikipedia page on the Skrulls states:

The first alien shapeshifters introduced in the Ultimate Marvel Universe are known on Earth as the Chitauri, who financed the Nazi regime in an attempt to conquer the Earth.
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Another race called Skrulls, physically resembling the Skrulls of the mainstream Marvel universe appear later ... ; these Skrulls dislike being confused with the Chitauri, whom they call terrorists.

The Skrulls in the main Marvel universe have similar capabilities as the Chitauri, as they're both invading aliens with shapeshifting powers. However, the Chitauri were created in the Ultimate universe as a distinct race from the Skrulls.

We already know that Loki is the big bad from the Asgardian villain team but who are the other baddies? Using the Cosmic cube first seen in Captain America, the First Avenger, Loki summons:

The Chitauri: Hailing from the Marvel Earth #1610 (also called the Ultimates Marvel Universe) the Chitauri are an alien race who believe themselves to be "the immune system of the Universe," attacking other species who do not possess a hive-mind intelligence and attempt to exercise free will. Confused with the Skrulls, both species possess similar shape-changing abilities.

Powers and Abilities: The Chitauri were able to mimic human form and absorb human knowledge, apparently by ingesting the bodies or brains of the humans they imitated. In their natural form, they appear to be large, and reptilian, but no clear images of their native form have been shown. Although the drone workers were nearly mindless and not much more durable than humans, the high-ranking officers such as Kleiser possessed super-strength to rival Captain America, superhuman stamina and durability, extremely rapid regeneration, and the ability to see or sense invisible objects or people.